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PubMed1986

Functional reconstitution of the alpha 2-adrenergic receptor with guanine nucleotide regulatory proteins in phospholipid vesicles.

We describe the successful reconstitution of functional interactions between an inhibitory adenylate cyclase-coupled receptor and various nucleotide-binding regulatory proteins in phospholipid vesicles. The receptor is the alpha 2-adrenergic receptor (alpha 2AR) which has been partially purified (approximately 500-5000-fold) from human platelet membranes. The nucleotide-binding regulatory proteins include purified preparations of human erythrocyte Ni and Ns, bovine retinal transducin and the recently discovered bovine brain No. Addition of the physiologic ligand, epinephrine, to vesicles containing the alpha 2AR and Ni results in stimulation of the GTPase activity in Ni. This stimulation of GTPase activity by epinephrine is prevented in the presence of the alpha-adrenergic antagonist, phentolamine, which indicates that a functional reconstitution of the alpha 2AR and Ni has been established. The maximum turnover number for the alpha 2AR-mediated epinephrine-stimulated GTPase activity in Ni is similar to the maximal turnover numbers obtained for the beta-adrenergic receptor-mediated isoproterenol-stimulated GTPase activity in Ns and the rhodopsin-mediated light-stimulated GTPase activity in transducin (0.5-1.5 mol of Pi released per min per mol of nucleotide regulatory protein). Functional similarities between the alpha 2AR and rhodopsin are observed in their interactions with the various nucleotide-binding regulatory proteins. Thus, both of these receptor proteins are capable of promoting the maximal activation of Ni and No while being much less effective in promoting the activation of Ns. However, there are differences between the alpha 2AR and rhodopsin in their interactions with transducin. Specifically, while rhodopsin will maximally activate transducin, the alpha 2AR is much less effective in promoting this activation (i.e. approximately 20% as effective as rhodopsin). Overall, these results suggest the following specificities of interaction: for rhodopsin, transducin approximately equal to Ni approximately equal to No much greater than Ns; while for alpha 2AR, Ni approximately equal to No greater than transducin greater than or equal to Ns.

Adenylyl Cyclases↗
PubMed1994

Expression of phototransduction cascade genes in the ground squirrel retina.

PURPOSE: This study describes the expression and distribution of phototransduction cascade gene products in the cone-dominant retina of the ground squirrel Spermophilus tridecemlineatus. METHODS: Messenger RNA expression was studied by blot hybridization, and the distribution of the gene products was investigated by immunocytochemistry. RESULTS: RNA blot hybridization showed messages for the alpha 2, beta 1, and beta 3 subunits of transducin but was negative for rhodopsin, alpha 1-transducin, and the alpha, beta, and gamma subunits of cyclic guanosine monophosphate (cGMP) phosphodiesterase. Immunocytochemical labeling indicated that the approximate ratio of the photoreceptor types in ground squirrel retina is 90.6% for green cones, 6.3% for rod-like cells, and 3.1% for blue cones. Rod-like cells were immunopositive for rhodopsin and blue opsin. All photoreceptor elements were labeled by antibodies against alpha 1-transducin (which recognizes both the alpha 1 and alpha 2 isoforms), beta 3-transducin, and the rod gamma subunit of phosphodiesterase, whereas no cells were labeled by antibodies against the rod alpha and beta subunits of phosphodiesterase or against the rod cGMP-gated cation channel. Rod-like cells and blue cones were stained by antibodies against beta 1-transducin. CONCLUSIONS: The authors demonstrate new cone-like traits in the biochemical make-up of rod-like cells, and a distribution of the transducin beta subunit in the ground squirrel is different from that found in other mammals.

Animals↗
PubMed1987

Differential distribution of signal-transducing G-proteins in retina.

We used specific antibodies in immunoblot studies of membrane fractions derived from bovine retina, and in immunohistochemical studies of sections of rat retina to determine the distribution of two guanine nucleotide binding proteins Go (a G-protein of unknown function discovered in the brain) and transducin, in retina. Both Go and transducin were readily detected in membranes derived from whole retina, and in crude rod outer segment membranes. Purification of rod outer segment membranes by sucrose density gradient centrifugation resulted in enrichment in transducin and depletion of Go immunoreactivity. Transducin-alpha immunoreactivity was localized to photoreceptor inner and outer segments and the outer nuclear layer. In contrast, Go-alpha immunoreactivity was localized in the inner and outer plexiform layers and ganglion cell layer. The results indicate that Go unlike transducin, is not associated with rod outer segment membranes and is therefore unlikely to function in phototransduction. Go is, however, relatively abundant in neural layers of retina where it may be involved in signal transduction.

Animals↗
PubMed1989

Calcium regulates the rate of rhodopsin disactivation and the primary amplification step in visual transduction.

The kinetics of the light-induced activation of transducin as well as the subsequent disactivation process can be monitored by means of a specific light scattering transient PA. In this communication it is demonstrated that the rate of transducin disactivation is calcium dependent, increasing when the calcium concentration is decreased. As a consequence of the accelerated recovery in low calcium, the time to the peak of the transducin activation process is shortened and the gain of the primary amplification step, i.e. the number of transducin molecules activated per bleached rhodopsin, is reduced. Experiments using hydroxylamine as an artificial quencher of rhodopsin activity suggest that calcium acts upon rhodopsin kinase and not upon the rate of the GTPase. This would indicate that calcium may control visual adaptation not only by regulating guanine cyclase activity, but also by affecting the primary step in the transduction cascade, the rhodopsin-transducin coupling.

Animals↗
PubMed2006

G-protein alpha and beta-gamma subunits interact with conformationally distinct signaling states of rhodopsin.

Light activated rhodopsin interacts with domains on all three subunits of transducin. Two of these domains, the C-terminal regions of the alpha and gamma subunits mimic the ability of transducin to stabilize the active conformation of rhodopsin, metarhodopsin II, but display different roles in transducin activation process. Whether the interactions are with the same or different complimentary sites on Meta II is unknown. We have used chemo-selective thioalkylation of rhodopsin and UV/visible spectroscopy to show that interactions with transducin C-terminal domains can be selectively disrupted. These data provide evidence that formal structural determinants on Meta II for these domains of transducin are different. In a set of complimentary experiments we examined the reactivity of Meta II species produced in the presence of the Gtalpha and Gtgamma subunit peptides to hydroxylamine. Analysis of the rates of Meta II decay confirms that the conformational states of Meta II when bound to Gtalpha and Gtbetagamma represent distinct signaling states of rhodopsin.

Animals↗
PubMed2001

A visual pigment expressed in both rod and cone photoreceptors.

Rods and cones contain closely related but distinct G protein-coupled receptors, opsins, which have diverged to meet the differing requirements of night and day vision. Here, we provide evidence for an exception to that rule. Results from immunohistochemistry, spectrophotometry, and single-cell RT-PCR demonstrate that, in the tiger salamander, the green rods and blue-sensitive cones contain the same opsin. In contrast, the two cells express distinct G protein transducin alpha subunits: rod alpha transducin in green rods and cone alpha transducin in blue-sensitive cones. The different transducins do not appear to markedly affect photon sensitivity or response kinetics in the green rod and blue-sensitive cone. This suggests that neither the cell topology or the transducin is sufficient to differentiate the rod and the cone response.

Ambystoma↗
PubMed1995

Constitutive activation of opsin: interaction of mutants with rhodopsin kinase and arrestin.

Mutation of Gly90, Glu113, Ala292, and Lys296 in the visual pigment rhodopsin constitutively activates the protein for activation of the G protein transducin. Three of these mutations have been shown to cause two different human diseases. Mutation of Gly90 and Ala292 results in complete night blindness, and mutation of Lys296 results in the degenerative disease retinitis pigmentosa. We show here that the mutants not only constitutively activate transducin but are also constitutively activated for phosphorylation by rhodopsin kinase. In addition, the phosphorylated mutants are shown to bind tightly to the inhibitory protein arrestin in a reaction that quenches the activity toward transducin. Thus the same mutations that result in constitutive activation of transducin also result in constitutive phosphorylation by rhodopsin kinase and binding of arrestin to inhibit the activity. This implies that the same conformational change may be responsible for activation of transducin and rhodopsin kinase. It also suggests that degeneration of photoreceptor cells in retinitis pigmentosa results indirectly from the activated state of the receptor, perhaps as a consequence of phosphorylation and persistent binding of arrestin.

Amino Acid Sequence↗
PubMed1981

Flow of information in the light-triggered cyclic nucleotide cascade of vision.

Photolyzed rhodopsin catalyzes the exchange of GTP for FDP bound to a protein in retinal rod outer segments. We previously proposed that the GTP complex of this protein regulates the cyclic GMP phosphodiesterase and that it may be the first amplified intermediate in visual excitation [Fung, B. K.-K. & Stryer, L. (1980) Proc. Natl. Acad. Sci. USA 77, 2500-2504]. We report here the identification and characterization of transducin, a regulatory protein consisting of three kinds of polypeptide chains: T alpha (39 kilodaltons), T beta (36 kilodaltons), and T gamma (approximately 10 kilodaltons). Reconstituted membranes containing transducin and rhodopsin but no phosphodiesterase exhibit GTPase activity and amplified binding of guanosine 5'[beta, gamma-imido]triphosphate (p[NH]ppG), a nonhydrolyzable analog of GTP, on illumination. A single photolyzed rhodopsin molecule led to the uptake of p[NH]ppG by 71 molecules of transducin. High-pressure liquid chromatography showed that the binding site for GTP is on the alpha subunit of transducin. The isolation of the complex of ;[NH]ppG with T alpha enabled us to determine whether this species is the activator of the phosphodiesterase. We found that phosphodiesterase on unilluminated disc membranes can indeed be fully activated by addition of T alpha containing bound p[NH]ppG. These findings strongly suggest that transducin is the first amplified information-carrying intermediate in the cyclic nucleotide cascade of vision.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1985

Antisera against a guanine nucleotide binding protein from retina cross-react with the beta subunit of the adenylyl cyclase-associated guanine nucleotide binding proteins, Ns and Ni.

Antisera were produced in rabbits against a guanine nucleotide binding protein (N protein), transducin, purified from bovine retina. Antiserum AS/1, which recognized all three subunits (alpha, beta, and gamma) of the holoprotein, was tested for cross-reactivity with the subunits of the adenylyl cyclase [adenylate cyclase; ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]-associated stimulatory (Ns) and inhibitory (Ni) N proteins purified from human erythrocytes. As/1 showed strong reactivity against the beta subunits of both Ns and Ni but failed to cross-react with either the alpha or gamma subunits of Ns and Ni. Seven additional antisera against transducin reacted with the beta subunits but not with the alpha or gamma subunits of Ns and Ni. A single antiserum against transducin reacted with the alpha subunit of Ni but not of Ns. Immunostaining of the beta subunits of Ns and Ni was proportional to the amount of beta subunit blotted and to the antiserum concentration. Immunostaining of either human erythrocyte or bovine cerebral cortical plasma membrane proteins with AS/1 showed a single band, comigrating with the beta subunit of transducin; this band was absent in bovine erythrocyte membranes. Estimation of the amount of beta subunit by immunoblotting with AS/1 showed that the beta subunit comprises approximately equal to 2% of bovine cerebral cortical plasma membrane protein, approximately equal to 100-fold more than in human erythrocyte membranes. These findings provide immunochemical evidence for similarities in the beta subunits and differences in the alpha and gamma subunits of this family of N proteins. Antisera against transducin react specifically with the beta subunits of Ns and Ni in crude plasma membranes and, thus, can serve as specific probes for the beta subunit.

Adenylyl Cyclases↗
PubMed1999

Blocking taste receptor activation of gustducin inhibits gustatory responses to bitter compounds.

Gustducin, a transducin-like guanine nucleotide-binding regulatory protein (G protein), and transducin are expressed in taste receptor cells where they are thought to mediate taste transduction. Gustducin and transducin are activated in the presence of bovine taste membranes by several compounds that humans perceive to be bitter. We have monitored this activation with an in vitro assay to identify compounds that inhibited taste receptor activation of transducin by bitter tastants: AMP and chemically related compounds inhibited in vitro responses to several bitter compounds (e.g., denatonium, quinine, strychnine, and atropine). AMP also inhibited behavioral and electrophysiological responses of mice to bitter tastants, but not to NaCl, HCl, or sucrose. GMP, although chemically similar to AMP, inhibited neither the bitter-responsive taste receptor activation of transducin nor the gustatory responses of mice to bitter compounds. AMP and certain related compounds may bind to bitter-responsive taste receptors or interfere with receptor-G protein coupling to serve as naturally occurring taste modifiers.

Adenosine Monophosphate↗
PubMed2001

Light- and guanosine 5'-3-O-(thio)triphosphate-sensitive localization of a G protein and its effector on detergent-resistant membrane rafts in rod photoreceptor outer segments.

Detergent-resistant membrane microdomains in the plasma membrane, known as lipid rafts, have been implicated in various cellular processes. We report here that a low-density Triton X-100-insoluble membrane (detergent-resistant membrane; DRM) fraction is present in bovine rod photoreceptor outer segments (ROS). In dark-adapted ROS, transducin and most of cGMP-phosphodiesterase (PDE) were detergent-soluble. When ROS membranes were exposed to light, however, a large portion of transducin localized in the DRM fraction. Furthermore, on addition of guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) to light-bleached ROS, transducin became detergent-soluble again. PDE was not recruited to the DRM fraction after light stimulus alone, but simultaneous stimulation by light and GTPgammaS induced a massive translocation of all PDE subunits to the DRM. A cholesterol-removing reagent, methyl-beta-cyclodextrin, selectively but partially solubilized PDE from the DRM, suggesting that cholesterol contributes, at least in part, to the association of PDE with the DRM. By contrast, transducin was not extracted by the depletion of cholesterol. These data suggest that transducin and PDE are likely to perform their functions in phototransduction by changing their localization between two distinct lipid phases, rafts and surrounding fluid membrane, on disc membranes in an activation-dependent manner.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed2000

The delta subunit of type 6 phosphodiesterase reduces light-induced cGMP hydrolysis in rod outer segments.

The delta subunit of the rod photoreceptor PDE has previously been shown to copurify with the soluble form of the enzyme and to solubilize the membrane-bound form (). To determine the physiological effect of the delta subunit on the light response of bovine rod outer segments, we measured the real time accumulation of the products of cGMP hydrolysis in a preparation of permeablized rod outer segments. The addition of delta subunit GST fusion protein (delta-GST) to this preparation caused a reduction in the maximal rate of cGMP hydrolysis in response to light. The maximal reduction of the light response was about 80%, and the half-maximal effect occurred at 385 nm delta subunit. Several experiments suggest that this effect was not due to the effects of delta-GST on transducin or rhodopsin kinase. Immunoblots demonstrated that exogenous delta-GST solubilized the majority of the PDE in ROS but did not affect the solubility of transducin. Therefore, changes in the solubility of transducin cannot account for the effects of delta-GST in the pH assay. The reduction in cGMP hydrolysis was independent of ATP, which indicates that it was not due to effects of delta-GST on rhodopsin kinase. In addition to the effect on cGMP hydrolysis, the delta-GST fusion protein slowed the turn-off of the system. This is probably due, at least in part, to an observed reduction in the GTPase rate of transducin in the presence of delta-GST. These results demonstrate that delta-GST can modify the activity of the phototransduction cascade in preparations of broken rod outer segments, probably due to a functional uncoupling of the transducin to PDE step of the signal transduction cascade and suggest that the delta subunit may play a similar role in the intact outer segment.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed2001

Regulation of photoreceptor phosphodiesterase (PDE6) by phosphorylation of its inhibitory gamma subunit re-evaluated.

Phosphorylation of the inhibitory gamma subunit (Pgamma) of rod cGMP phosphodiesterase (PDE6) has been reported to turn off visual excitation without the requirement for inactivation of the photoreceptor G-protein transducin. We evaluated the significance of Pgamma phosphorylation for PDE6 regulation by preparing Pgamma stoichiometrically phosphorylated at Thr(22) or at Thr(35). Phosphorylation of Pgamma at either residue caused a minor decrease--not the previously reported increase--in the ability of Pgamma to inhibit catalysis at the active site of purified PDE6 catalytic dimers. Likewise, Pgamma phosphorylation had little effect on its potency to inhibit transducin-activated PDE6 depleted of its endogenous Pgamma subunits. The strength of Pgamma interaction with the regulatory GAF domain of PDE6 was reduced severalfold upon Pgamma phosphorylation at Thr(22) (but not Thr(35)), as judged by allosteric changes in cGMP binding to these noncatalytic sites on the enzyme (Mou, H., and Cote, R. H. (2001) J. Biol. Chem. 276, 27527-27534). In contrast, the effects of Pgamma phosphorylation on its interactions with activated transducin were much more pronounced. Phosphorylation of Pgamma at either Thr(22) or Thr(35) greatly diminished its ability to bind activated transducin, consistent with earlier work. In situ phosphorylation of Pgamma by endogenous rod outer segment kinases was enhanced severalfold upon light activation, but only approximately 10% of the endogenous Pgamma was phosphorylated. This is attributed to Pgamma being a poor substrate for protein kinases when associated with the PDE6 holoenzyme. We conclude that, contrary to previous reports, Pgamma phosphorylation at either Thr(22) or Thr(35) modestly weakens its direct interactions with PDE6. However, Pgamma phosphorylation subsequent to its dissociation from PDE6 is likely to abolish its binding to activated transducin and may serve to make phosphorylated Pgamma available to regulate other signal transduction pathways (e.g. mitogen-activated protein kinase; Wan, K. F., Sambi, B. S., Frame, M., Tate, R., and Pyne, N. J. (2001) J. Biol. Chem. 276, 37802-37808) in photoreceptor cells.

Animals↗
PubMed2001

RGS9-G beta 5 substrate selectivity in photoreceptors. Opposing effects of constituent domains yield high affinity of RGS interaction with the G protein-effector complex.

RGS proteins regulate the duration of G protein signaling by increasing the rate of GTP hydrolysis on G protein alpha subunits. The complex of RGS9 with type 5 G protein beta subunit (G beta 5) is abundant in photoreceptors, where it stimulates the GTPase activity of transducin. An important functional feature of RGS9-G beta 5 is its ability to activate transducin GTPase much more efficiently after transducin binds to its effector, cGMP phosphodiesterase. Here we show that different domains of RGS9-G beta 5 make opposite contributions toward this selectivity. G beta 5 bound to the G protein gamma subunit-like domain of RGS9 acts to reduce RGS9 affinity for transducin, whereas other structures restore this affinity specifically for the transducin-phosphodiesterase complex. We suggest that this mechanism may serve as a general principle conferring specificity of RGS protein action.

3',5'-Cyclic-GMP Phosphodiesterases↗
PubMed1999

Photoreceptor cell damage by light in young Royal College of Surgeons rats.

PURPOSE: To determine the effects of genetic background and light rearing conditions on intense-light-mediated retinal degeneration in young RCS rats. MATERIALS AND METHODS: Albino rats, homozygous or heterozygous for the rdy gene were bred and born in dim cyclic light. At P7 they were moved to a dark environment, and maintained there until exposure to intense visible (green) light at P18 or P25. Other rats remained in the dim cyclic light environment. At various times between P11 and P40 rats were killed for determinations of rhodopsin and photoreceptor cell DNA levels, western transblot analysis of retinal S-antigen (arrestin) and alpha-transducin, or northern slot blot analysis of their respective mRNA levels. RESULTS: At P18, unexposed dark maintained homozygous RCS rats and their phenotypically normal heterozygous counterparts have nearly equivalent rhodopsin levels and photoreceptor cell DNA. Intense light exposure at this age, to 8 hours of continuous light or 3 hours of intermittent light, did not lead to a loss of either rhodopsin or retinal DNA when compared with their respective unexposed controls. At P25 rhodopsin levels were higher than at P18, while photoreceptor cell DNA was essentially the same as in the younger rats. However, intense light exposure at P25 resulted in substantial losses of rhodopsin and photorecptor cell DNA and the losses were greater in homozygous rats than in heterozygous animals. Light damage of P25 rats maintained in dim cyclic light was essentially the same as in dark maintained homozygous rats, but no damage was found in the heterozygous animals. By western analysis, alpha-transducin levels in the retina increased with time in darkness, while retinal S-antigen levels either remained the same or decreased during the period P15-P35. For rats in the cyclic light environment S-antigen expression was greater than alpha-transducin at all ages. Slot blot analysis of mRNAs for the two proteins generally followed the patterns seen by western analysis. S-antigen mRNA was expressed at an earlier age and at higher levels than alpha-transducin in both types of rats from both light rearing conditions. Peak expression of S-antigen most often occurred at P18 in both the heterozygous and homozygous rats. CONCLUSIONS: The relative expressions of S-antigen and alpha-transducin in P18 and P25 rats correlates with their relative resistance to retinal light damage at P18 and their enhanced susceptibility at P25. Rats homozygous for the rdy gene also exhibit more damage than heterozygous animals when photoreceptor cell DNA is used to estimate the extent of retinal light damage.

Aging↗
PubMed1990

Cyclic GMP and photoreceptor function.

A single photon can be detected by a rod photoreceptor cell. The absorption of light by rhodopsin triggers a cascade of reactions that amplifies the photon signal and results in ion channel closure with hyperpolarization of the rod photoreceptor cell. Light-induced conformational changes in rhodopsin facilitate the binding of a guanosine nucleotide-binding protein, transducin, which then undergoes a GTP-GDP exchange reaction and dissociation of the transducin complex. A subunit of transducin then activates a phosphodiesterase complex that hydrolyzes cyclic GMP. In darkness, cyclic GMP binds to cation channels of the photoreceptor plasma membrane, maintaining them in an open configuration. The light-induced reduction in cyclic GMP concentration dissociates the bound cyclic GMP, resulting in channel closure and hyperpolarization. Down-regulation of the cascade involves other proteins that block the interaction of transducin with rhodopsin and another protein that may interfere with transducin recycling. Cone photoreceptors possess a light-activated cascade that follows the rod format, but it is composed of proteins that are homologous to those of rod photoreceptors. Phototransduction in invertebrate photoreceptors uses rhodopsin to activate a cascade that uses phosphoinositides and calcium ion to regulate membrane polarization.

Antigens↗
PubMed1997

Phototransduction cascade and circadian oscillator in chicken pineal gland.

The chicken pineal gland has an endogenous circadian oscillator that controls the diurnal oscillation of N-acetyltransferase activity responsible for melatonin rhythm. It has been speculated that the chicken pineal cell contains a photoreceptive molecule that receives the environmental light signal and transmits the signal to the oscillator for resetting the phase. In spite of several lines of evidence suggesting the similarity between retinal and pineal photon-signal transducing proteins, the identity of the photoreceptive molecule had been an open question. In 1994, we isolated a pineal cDNA encoding a novel photoreceptive molecule and named it "pinopsin." The protein expressed in 293EBNA cells bound 11-cis-retinal to form a blue-sensitive pigment with an absorption maximum at about 470 nm. A putative G-protein interaction site of pinopsin shared a relatively high similarity in amino acid sequence to that of rhodopsin, implying that pinopsin functionally couples with transducin or transducin-like G-protein(s) in the pineal cells. We have cloned a cDNA for chicken pineal transducin alpha-subunit, and the deduced amino acid sequence contained a potential site to be ADP-ribosylated by pertussis toxin (PTX). Therefore, the transducin-mediated pathway could be blocked by PTX, though previous studies showed that treatment of the cultured chicken pineal cells with PTX had no effect on the light-induced phase-shift of the oscillator. Accordingly, it is unlikely that transducin mediates the light-input pathway to the oscillator, which may involve PTX-insensitive G-protein(s) or some unidentified component(s). The G-protein coupled receptor-mediated signaling processes regulating melatonin synthesis are discussed.

Adenosine Diphosphate Ribose↗
PubMed1987

Deactivation of photoactivated rhodopsin by rhodopsin-kinase and arrestin.

Photoactivated rhodopsin (R) catalyses, by repetitively interacting with many copies of a guanosine nucleotide binding protein (transducin), the amplified binding of GTP to transducin molecules which then activate cyclic GMP phosphodiesterase. Electrophysiologists recently have shown that cyclic GMP keeps ion channels in the plasma membrane of the rod outer segment open in darkness, and that light-induced hydrolysis of cyclic GMP leads to closure of the channels and therefore to hyperpolarization of the rod cell. Photoactivated rhodopsin interacts not only with transducin, but with two more proteins: a protein kinase that specifically phosphorylates R (in contrast to dark-adapted rhodopsin) at multiple sites; and an abundant soluble protein of 48 KDal (called 48 K-protein, S-antigen, or arrestin) that specifically binds to phosphorylated R. Phosphorylation partially suppresses the ability of R to catalyze transducin-mediated phosphodiesterase activation even in the absence of arrestin. Binding of arrestin to the phosphorylated R potentiates this inhibitory effect, most probably because arrestin competes with transducin for binding on the phosphorylated R. Phosphorylation, in conjunction with arrestin binding, therefore appears to be a mechanism that terminates the active state of the receptor, R.

3',5'-Cyclic-GMP Phosphodiesterases↗